1. Field of the Invention
The present invention relates to a bending method and a mandrel for a multi-layered pipe including a double pipe, and more specifically to a bending method and a mandrel for a multi-layered pipe wherein bending work is performed with an inside mandrel inserted into an inner pipe of the multi-layered pipe, in which the inner pipe and an outer pipe are disposed concentrically with an appropriate gap therebetween, and an outside mandrel inserted into the gap between the inner pipe and the outer pipe positioned on the outside thereof and the gaps between the outer pipe positioned on the inside and outer pipes positioned on the outside thereof.
2. Description of the Related Art
A double pipe used as an exhaust pipe in an automobile or the like, for example, comprises an inner pipe and an outer pipe disposed with an appropriate gap therebetween, and is bent by a bending operation in a predetermined direction and to a predetermined angle in order to emulate the constitution of the vehicle body, avoid other components, and so on.
This type of double pipe must be bent while maintaining the gap between the inner pipe and outer pipe, and hence the pipe is bent following the insertion of an inside mandrel into the inner pipe and an annular outside mandrel into the gap between the inner pipe and outer pipe (see Japanese Examined Patent Application Publication S55-24971, Japanese Unexamined Patent Application Publication H9-155456, and Japanese Unexamined Patent Application Publication 2001-269721, for example).
In the techniques disclosed in Cited Documents 1 and 2, the outside mandrel is formed as a tube. Hence the outside mandrel cannot change shape and the distal end thereof can only be inserted up to the starting point of the bending portion of the pipe. As a result, a stable double pipe with a constant gap between the inner pipe and outer pipe cannot be obtained at the bending portion.
In the technique disclosed in Cited Document 3, the outside mandrel is constituted by two strip-form elastic plates and bending work is performed with the distal ends of the outside mandrels within the bending range of the pipe. According to this technique, the gap between the inner pipe and outer pipe is maintained. In this technique, the distal end portions of the two outer mandrels must be positioned correctly in an inside R portion and an outside R portion. However, the length of the outside mandrels corresponds to the length of the double pipe, and thus the outside mandrels may contort, causing deviation in the pitch of the outside mandrels such that the positions of the outside mandrels deviate from a predetermined position during the bending work. In particular, when the double pipe is rotated continuously to perform bending work in different directions, the pipe must be twisted, as a result of which the outside mandrels often become contorted.
It is therefore an object of the present invention to provide a bending method and a mandrel for a multi-layered pipe according to which the multi-layered pipe can be bent while maintaining a stable gap between an inner pipe and an outer pipe and between outer pipes even when the pipe is continuously bent into different directions.